Coaxial cable and method of making same



May 20, 1947. A. z. MAMPLE COAXIAL CABLE A'ND METHOD OF MAKING SAME`Filed Nov. 26, 1942 4 sheets-sheet 1 INVENTOR A Z MAMFLE ATTORN EY May20, 1947- A. z. MAMPLE COAXIAL CABLE AND METHOD OF MAKING SAME FiledNov. ze, 1942 4 Sheets-Sheet 2 d om May 2o, 1947. A Z, MAMPLE 2,420,712-

COAXIAL CABLE AND METHOD 0F MAKING SAME Filed Nov. 26, 1942 4Sheets-Sheet 3 W-MWNNWAV Avv Av Av ATTORNEY May 20, 1947-y vA. z. MAMPLE2,420,712

` GOAXIAL CABLE AND METHOD 0F MAKING SAME Filed Nov. 26, 1942 4Sheets-Sheet 4 fh i nnmuun, C w

INVENTOR A.z. MAM PL:

ATTORN EY Patented May 20, 1947 COAXIAL CABLE AND METHOD`OF MAKING SAMEAdolph Z. Mample, Glen Rock, N. J., assigner to The Western UnionTelegraph Company, New York, N. Y., a corporation of New YorkApplication November 26, 1942, Serial No. 467,001

10 Claims.

My invention relates to a conducting system particularly adapted for thetransmission of a wide band of frequencies Well above the frequencies atpresent employed in carrier transmission and whose upper range may be inmegacycles, and more particularly to an improvement in the method ofbuilding or constructing a coaxial cable such as disclosed in my U. S.Patent No. 2,293,155, issued August 18, 1942, and to the improved cableconstructed in accordance with the method disclosed herein.

The development of Wide band transmission circuits has resulted in aform of circuit, generally referred to as the coaxial or concentriccircuit, comprising an outer conducting tube which encloses a centralconductor that is spaced from the outer tube by means of insulatingdiscs. 'Ihe high frequency transmission circuit is formed between theinner surface of the outer conductor and the outer surface of the innerconductor; the outer tubing serves both as a conductor and a shield.

In a coaxial cable system, if the two conductors be separated in so faras is possible by a dielectric consisting largely of air or othergaseous medium, the cableI has a number of desirable characteristics.Its attenuation at all frequencies is quite low as compared with thecorresponding attenuation of open wire lines and cable circuits such asare commonly employed for telephone and telegraph transmission. Thecoaxial cable may, therefore, be employed for the transmission of muchwider bands of frequencies than is possible with types of Atransmissioncircuits heretofore used, and it lalso has the advantage of beingsubstantially free from interference from neighboring conductor systemsand in itself tends to produce but little interference into adjacenttransmission circuits. In order to maintain the conductors in theirrelative concentric positions, it is necessary to provide the insulatingwashers above mentioned, but the presence of such Washers increases theattenuation, and if the washers are spaced close together as hasheretofore been found necessary in practice where the cylindricalconductors of the cable are of flexible construction, the amount ofsolid dielectric material per unit length of the cable may be increasedto such an extent that the resultant increase in attenuation becomeshighly undesirable.

It has heretofore generally been deemed necessary, because of theproblems of installation,

to provide a coaxial cable that could be rolled up telephone and.telegraph cables. The steps of manufacturing coaxial cables, reeling iton a cable reel, transporting it to the place of installation,unrecling, straightening out and pulling it into place, all subject thecable to various stresses such as tension, straightening, bending,compression and the like. Bending tends to make the tubular, orcylindrical, outer conductor become generally elliptical, placing theouter periphery in tension and the inner periphery in compression, whileat the same Vtime placing the inner conductor in tension, and thisintroduces a lack of symmetry into the coaxial system, and attemptsheretofore to straighten the cable after having thus been bentintroduced irregularities in the cable structure, particularly in theinner conductor, which substantially increases the attenuation of thecircuit, makes the circuit more susceptible to external interference,and causes the values of the circuit to vary with movement of the cablesuch as in a wind. Also, any slight irregularity in the structure of thecable will make itself felt by a variation in its local characteristicimpedance and cause reection or echo effects and thus cause variation inthe output impedance of the cable.

In an attempt to avoid lack of symmetry and irregularities in thestructure ofthe cable, as stated in my aforesaid patent, the art inpractice had resorted to a very close spacing of the insulating Washers,and since it was found impracticable, if not impossible, to draw theouter conductor over the washers when thus closely spaced it was thepractice to form thel outer conductor by winding filamentary orpreformed strips in a long spiral around the spacing Washers, and insome cases weaving the outer conductor around theA washers. Forming theouter conductor in this manner enabled manufacture of such a cable, butit has its mechanical limitations because it may readily becomeelliptical when bent, and also has introduced therein electricaldisadvantages. Filamentary or strip construction is not satisfactoryelectrically because effective shielding fundamentally involves threeseries of loops whose planes are mutually perpendicular to each other,each of which loops could be replaced by a sheet metal to form a cube.Thus, in a shield for a conductor it involves a combination of closedloops in series Whose axes run along the center of the conductor. It isevident that these loops should be uniform in resistance and ofsuiciently low resistance to be an eiective shield. In a filamentaryspiral, spiral strip or woven outer conductor, the shielding loops mustrely on the con- 3 tact resistance of the iilamentary or spiral stripsto form closed low resistance loops, and nearly all metals which wouldbe practicable for use as an outer conductor, such as copper, form nlmson the surface which cause high resistance contacts. Similarly it isknown that the wires forming stranded cable often vary appreciably inresistance even when adjacent wires are measured. It is evident,therefore, that lamentary or strip construction. -of the outer conductorin which the components parts of the loop merely-touch inthe spiral donot provide loops which are uniformly low in resistance, and thereforesuch construction does not provide the most effective shielding.

Another disadvantage of the spiralor filamentary types of coaxial cablesas heretofore proposed or employed is the excessively high cost of suchcables, due principally Lto the involved methods of manufacturing andinstalling the cables, and also due to the necessity for close spacingof repeatersbecause of an undesirably high attenuation in the cable dueto close spacing of the insulating washers, irregularities in the cablestructure, and the like.

An object of the invention is an improved and practicable method ofconstructing or fabricating a coaxial cable in which the foregoingdisadvantages are obviated or substantially minimized.

Another object of the' invention is an improved coaxial cable thatclosely approximatesin practice the perfect theoretical cableelectrically and mechanically when the cable is installed in place.

A further object is an improvement in a coaxial cable of the typedisclosed in my aforesaid patent, whereby less care is necessary in thetransportation and handling of the cable or its component parts prior toor during the installation of the cable in'its permanent location, and

greater latitude is permissible in regard to thel place of assembly ormanufacture of the cable and its component parts.

An additional object is an improved method for removing bends from theouter tubular conductor of a coaxial cable which, if permitted toremain, would introduce distortion in the coaxial circuit.

Still another object is an improved method of splicing the innerconductor of a coaxial cable to prevent any change in the diameter orconfiguration of the conductor at the spliced sections thereof.

Additional objects and advantages of the invention will be apparent fromthe following detailed description,`taken in connection with theaccompanying drawings, in which:

Fig. 1 is 9. longitudinal sectional view of a coaxial cable constructedin accordance with the present invention; t

Fig. 2 is a view of the inner conductor and spacing washers priortotheir insertion in the outer tubular conductors; y

Fig. 3 illustrates how a lack in symmetry in a coaxial system isintroduced when the cable is bent or deflected;

Fig. 4 illustrates how kinks or other irregularities are formed in theinner conductor .when the cable is straightened after having been bent;

Fig. 5 illustrates one of the initial steps of making or fabricating acoaxial cable in accordance with the present invention, when employed asan underground cable;

Fig. 6 shows how any bends present in the outer tubular conductor areremoved;

Fig. '7 is a view of a device employed to straighten out or remove anykinks or irregularities present in the inner surface of the outertubular conductor;

Fig. 8 illustrates the step of pulling in the central conductor andinsulating washers through the outer conductor when employed as anunderground cable; l

Fig. 9 illustrates the step' of pulling in the outer conductor in themethod of making or fabricating the cable when employed as an aerialcable;

Fig. 10 shows how bends present in the outer tubular conductor of theaerial cable are removed;

Fig. 11 shows the step of pulling the inner conductor and insulatingwashers into the outer conductor of the aerial cable;

Fig. 12 is a view of a preferred form of a tool inserted in the outerconductor for removing bends present in the outer conductor in themanner shown in Figs. 6 and l0, the tool being shown in grippingengagement with the conductor;

Fig. 13 shows the tool of Fig. l2 in its released position after theconductor has been straightened;

Fig. 14 is a cross-sectional view taken along the line H-II of Fig. 13;and

Fig. l5 shows a preferred method of splicing the outer and innerconductors of the cable.

Referring particularly to Fig. l of the drawings, there is shown acoaxial cable C constructed in accordance with the present invention.The outer tube 60 may be formed from a piece of seamless metal tubing,such as the flexible soft drawn copper tubing commercially manufacturedand used for water lines for refrigerators, and the like. One size ofsuch tubing which is suitable for the cable of the present invention hasan outside diameter of approximately 0.625 inch and an inside diameterof approximately 0.545 inch, although various other sizes may of coursebe employed. While generally manufactured in short lengths of forty tofifty feet, this tubing may be obtained from various tube manufacturersin lengths of from five hundred to six hundred feet and thus providesections sufficiently long for the practicable construction of thecable, such lengths being adapted to reach between manholes in the caseof underground cables and to reach a number of spans in the case ofaerial cables supported on pole lines in the usual manner. This tubingis comparatively inexpensive and does not require any special procedurein its manufacture except that it is drawn into longer lengths thanusual. The inner conductor 5l of the cable may be a copper wire such.for example, as a No. 6 B. 8: S. gauge wire, preferably soft drawn ormedium hard drawn, which has an outside diameter of approximately 0.162inch, such size in hard drawn wire being commonly employed for telegraphcircuits carried on pole lines. This wire is also comparativelyinexpensive and represents a well-known commercial product obtainablefrom various wire manufacturers. The inner conductor is supported by andspaced from the outer conductor by means of thin insulating pieces,referred to hereinafter as washers, made of polystyrene, or a siliceousclay-like material such as Isolantite or other suitable low lossinsulating material, the washers preferably being one-sixteenth of aninch thick. The spacing of the washers longitudinally of the cable mayvary, depending upon the size of the coaxial cable and other factors,and in the illustrative embodiment disclosed may be spaced from four toeight inches apart.

As shown in Fig. 2, the inner conductor l and washers 52 are assembled,the lwashers being maintained in proper spaced relation longitudinallyof the conductor by being cemented thereto or by very small beadedportions formed on the wire 5l adjacent to the washers, prior to theinsertion of the inner conductor in the outer tube 50. If beadedportions are employed, they preferably are of the order of 0.005 inch,and thus are too small to be drawn to scale in the figure, and do notintroduce irregularities in the structure of the conductor. When theinner conductor is thus formed, it may be reeled on a cable reel,preferably of large diameter, and transported to the place where thecable is to be built or fabricated. The outer tube 50 may similarly bewound on a cable reel of relatively large diameter and transported toIthe place where it is to be installed.

As set forth in U. S. patent to Green, No. 1,859,390, issued May 24,1932, Where the insulating washers that support the inner conductor arespaced a considerable distance apart and are .themselves relativelythin, the amount of solid dielectric material present per unit ofconductor length is so small as to produce substantially negativeincrease in attenuation over that which would be present if the entiredielectric space were gaseous. If, as Stated in the patent, it isnecessary for mechanical or other reasons to closely space the washersas, for example, where the inner and outer conductors are flexible, theresultant increase in attenuation may be so great as to render thesystem impractical as compared with former types of transmissionsystems.

In practice, however, because of mechanical or other reasons, it hasheretofore been found necessary in lamentary or spiral construction toprovide very close spacing of the washers, as close as three-quarters ofan inch in a cable which has recently been installed. This undesirablyclose spacing of the Washers was required because otherwise, when theouter conductor of a coaxial cable was subject to variations due tomanufacture, and to reeling, unreeling, transporting, and the like, theconductors would develop irregularities or 4become asymmetrical. Thiswill be appreciated from a consideration of Fig. 3 of the drawings, theouter tube being shown as fabricated from spirally laid metal strips.Bending of the cable due to manufacture, reeling, shipping, unreeling,straightening, and installing the same, subjects a cable to variousstresses such as tension, bending, etc. Bending tends to make thetubular outer conductor become elliptical, placing the outer peripheryin tension and the inner periphery in compression,

While at the same time placing the inner coneffects.

subject to distortion from external disturbing fields. In Fig. 3 theline A-A indicates a place where the inner conductor 5I is properlymaintained in its central position with respect to the outer tube 50 bymeans of the insulating piece 52. The external force produces a field inwhich the lines of force cutting thetwo concentric conductors producedifferences in potential between points of the two conductors, but sincethe same number of lines of force cut the two conductors on the oppositesides of the central conductor the induced potentials produce currentflows which are equal and opposite to each other so that a balance isobtained. Thus, 'where the conducting system is symmetrical with respectto the cutting lines of force, all differences in potential producedbetween any other two points of the two conductors will be balanced bysimilar differences of potential introduced in corresponding points onthe other side, and no interfering effects are caused-by externalfields.

When, however, the central conductor 5| is 4 displaced with respect tothe outer conductor 50, as indicated by the line B-B in Fig. 3, thelines of force cutting the` two conductors produce unequal electromotiveforces between the points, and the unequal induced potentials exertinterfering effects on the conducting system.

Also, when the outer tube 50 becomes flattened at points, as at theplace indicated by the line C-C in-Fig. 3, caused by bending due tomanufacture, reeling, shipping, unreeling, and installing, this causesthe outer conductor to become flattened on one side or elliptical, andcauses the inner conductor to be brought closer to the inner surface ofthe outer conductor, and then the system becomes asymmetrical. Thenature of these faults varies the capacity of the cable and introducesvariable induced potentials into the system, thus tending to destroy oneof the principal advantages of coaxial cable systems over other systems.Any change in the symmetry of the coaxial system is also reflected inanother phase of an important factor of the cable, namely, attenuation.This is due to the fact that the closer the inner conductor is to theouter conductor, the greater will be the attenuation due to capacity,and the chords formed by the inner conductor shown in Fig. 3 and thetendency of the outer conductor to become flattened on one side orelliptical, bring the inner conductor closer to the inner surface of theouter conductor, and the higher the capacity the greater the attenuationof the cable.

Fig. 4 illustrates how irregularities may be introduced into the centralconductor 5| when the coaxial cable is straightened after having beenbent during manufacture and installation in accordance with priormethods. When the cable is bent, the inner conductor 5| was placedundertension and caused to form chords between adjacent insulating pieces,and when the outer tube is straightened, due to the fact that the innerconductor cannot readily slip through the various Washers through whichit passes because of the binding effect of the washers, it tends to bowor have kinks formed therein at various places along the cable, one ofthese kinks being indicated at k in Fig. 4. The direction which the kinkmay take, of course, is not determinable, but in any event it causesirregularities in the inner conductor and results in reflection, thuscausing echo The relatively large displacement of the inner conductor,as shown in the figure, results from the cumulative effect of thestraightening of the various adjacent chords of the conductor. Thevarious irregularities and the disturbing eiects resulting therefrom areset forth in greater detail in my aforesaid Patent 2,293,155, thedisclosure of which is incorporated herein by reference.

Figs. 30, 31 and v32 of my aforesaid patent graphically indicate theincrease in leakage, capacity and attenuation, respectively, caused by adecrease in washer spacing in a co-axial cable. Referring to Fig. 30 ofthe patent, the graph illustrates the increase in leakage expressed inmhos -s per loop mile, with a reduction in washer spacing expressed ininches. It will be noted that, as the washer spacing decreases, andparticularly as the spacing becomes less than approximately two inches,the leakage increases enormously, and with a washer spacing ofthreequarters of an inch, which has heretofore been considered necessaryin practice, particularly where the outer conductor is composed of a la`mentary strip or is woven, the leakage becomes excessively high.

Fig. 31 of the aforesaid patent illustrates the increase of capacity,expressed in faradsX 104- per loop mile, and shows the excessively highincrease in the capacity of the cable with the close spacing between thewashers heretofore found necessary in the construction of coaxialcables. Fig. 32 thereof shows the vincrease in attenuation per mile ofcable with close washer spacing. In each of the graphs referred to, itwill be seen that, if the washer spacing can be increased so that is ofthe order of approximately 4 inches or more, the leakage, capacity andresulting attenuation of the cable are reduced to a value that greatlyimproves the electrical characteristics of the cable.

Due to the high frequencies employed in coaxial cable systems, the eiectof cable irregularities on the propagation of signals is of great impor-I the figure, and to remove such bends the tubular tance. Theseirregularities may result from even slight variations in the cablestructure and, as hereinbefore stated, may be introduced either duringmanufacture or installation. Initially, a slight irregularity in thestructure of either the outer or inner conductor of the cable will makethis apparent by variation in its local characteristic impedance andcause reflection. There thus results a series of attenuated waves whichare reflected and which return to the origin of the cable, manifestingthemselves by a complex echo and by 4a variation of the input impedance.A second possible effect consists in a double redection at two points ofirregularity. Here a wave is produced and superimposed at the outputside of the direct wave, giving rise to a prolongation of the signal,referred to as the signal tail or tailing of the signal. f ASuchirregularities in the cable structure vcause loss of energy by reection,whence an increase in the mean attenuation results and also distortionor interruption of the signals, which effects increase with thefrequency and therefore becomes increasingly important at the higherfrequencies at which coaxial systems are intended to be used.

Figs. 5 to 8 illustrate a method of building or fabricating anunderground coaxial cable in accordance with the invention. Referring toFig. 5, the preformed outer tubular conductor 50 is unwound from a cablereel 60 mounted on trunnions Bi and is pulled through an undergroundcable duct 83 between adjacent manholes 02, a`

bell-mouthed guide member 64 being inserted into the duct to facilitateentry of the cable therein.

conductor is stretched and thereby straightened by applying a tensionalforce thereto, as shownv in Fig. 6. Referring to the latter figure, theends of the tubular conductor projecting into adjacent manholes B2 haveinserted therein a pulling tool 68. This tool expands in the end of thetube and ilrmly grips the vsame while a pulling force is applied to thetool, as hereinafter explained in detail. Each tool has a pulling eye 81which is connected by means of a link Il to the ratchet chain hoist orother suitable pulling device 10. When the ratchet handle 1| of thepulling device 10 is reciprocated, this causes a length of chain or wirerope 12 to be taken up by the pulling device. At one end of the chain orrope is secured a shackle 1I which is inserted in a cable pulling-iniron 14, the cable iron being rmly embedded in the concrete comprisingone of thewalls of the manhole 62. A wooden plank or other suitablemeans 15 is employed to raise the chain or rope 12 in order to insure a.straight pull on the tube 50.

A very large'tensional force can thus be imposed on both ends of thetube 50, at the adiacent manholes, the amount of the force requiredgenerally being dependent upon the length of the tubular conductorbetween the manholes. Where lengths of from 400 to 600 feet of tubularconductor are thus straightened, the tensional or pulling force requiredmay be of the order of 1,000 pounds applied at each end, and thistensional force stretches the tubular conductor il slightly and to theextent necessary to pull out any bends which were introduced therein.This means that, when the inner conductor subsequently is pulled intothe tube 50, the spacing between the inner conductor and the tube l0will not vary due to curved portions or bends in the tube. Thestraightening or stretching operations also means that less care isrequired in the handling, shipping and pulling-in operation and enablessmaller reels to be employed, since whatever curvature is introducedinto the tubing by reason of being coiled on such a reel is effectivelyremoved.

After the tube 50 is pulled in place and straightularities, such as thedent indicated at b in Fig. 7, are removed by pulling a three-ballhammer through the tube 50, the tube having a bellmouthed captemporarily inserted thereon t0 prevent bending or kinking of the end ofthe tube and also to facilitate further steps in the fabrication of thecable. The three-ball hammer comprises a middle ball or mandrel 8ihaving conical-shaped ends, the member being slidably mounted on a rodor wire I2, the member 8l being struck, when necessary, by the two endballs or hammers 83 and 04, which are fastened securely to the pull rodor rwire 82. When the three-ball hammer comes in contact with anirregularity b, the rod or wire 82 and the balls 83 and I4 thereon aredrawn back and forth by the installers, producing a striking motion by'shown in the figure.

the member 8l against the irregularity b which is thus hammered andsmoothed out to reform the tubing. Where the tube is not deformed themandrel may be passed through quite easily, but where it is deformed themandrel will reform it into a tube of true circular cross-section. Thelargest diameter of the member 8| is but a few thousanclths of an inchless than the normal inside diameter of the tubing 50. After thethree-ball hammer is thus worked through the tube 50 from one manhole tothe next manhole, the inside of the inner diameter of the tube 50 willbe uniform throughout the length thereof and all elliptical sections,buckles, dents, etc., in the tubing will have been removed. The tube 50is now ready for the insertion of the inner conductor and spacingwashers.

Fig. 8 shows how the inner conductor 5l and the spacing washers or discs52 are pulled from a reel 88 into the outer tube 50. With a washerspacing of five inches, in a cable of the size hereinbefore stated, apulling force of the order of 20 lbs. per hundred feet of cable isnecessary to overcome the friction between the insulating discs and theouter tubular conductor, and even though lengths of from 500 to 600 feetof the inner conductor are thus pulled in, thev total pulling forcerequired does not appreciably elongate the copper wire comprising theinner` conductor.

After the inner conductor has been pulled in throughthe tube extendingbetween .adjacent manholes, irregularities in the wire l are removed byemploying a suitable stra'mhtening device for placing the wire under thenecessary tension, such a device being shown in Fig. 6 herein, althoughother devices may be used for this purpose, such as the device shown inFig. 23

of my aforesaid patent. The arrangement of Fig. 6 herein may be employedas shown in this iigure, except that a wire gripping and pulling device,which may be of a type well known in the art, such as a Buffalo grip orcome-along, is employed in lieu of the tube gripping device 66 Theamount of stretching force applied to the inner conductor to straightenit, however,` is not nearly so great as that required for straighteningthe tube 50. Where the inner conductor is, for example,'from 400 to 600feet long, a stretching force of the order of 300 pounds will usually befound sufficient to remove any kinks or irregularities which may havebeen introduced into the wire. Instead of employing two stretchingdevices, as shown in Fig. 6, one end of the inner conductor 5i may beanchored in any suitable manner, as to the pulling-in iron '14, at oneend of the manholes, and the stretching operation effected at theadjacent manhole, although in the case of long sections of cable, itwill usually be found desirable to employ straightening devices at bothmanholes.

inasmuch as the inner diameter of the outer tube 50 now represents atrue circle throughout the length thereof, and the conductor 5i isstraight and true, and the washers 52 are perT fectly concentric with ancuter diameter of only a few thousandths of an inch less than the innerdiameter of the outer conductor 5G, this causes the inner conductor tobe uniaxial with respect to the outer conductor and thus result in asymmetrical coaxial cable. Y

As above stated, the outer conductor 50 is a perfectk circular unitconductor, and it will be apparent that with no unequal tension on theinner conductor 5I with relation to the outer conductor, there are noforces tending tov form chords from washer to washer or to pull theinner conductor out of alignment with the axis of the-outer conductor.The symmetrical cable thus formed will thus permit of no induceddisturbing potentials, and no increases or variations in attenuation dueto capacity because of unequal spacing of the inner conductor withrelation to the outer conductor, and eliminates the undesirable echoeffects. It will further be evident that'the perfect condition of t? vecom.. pleted coaxialA cable obtained does not require extra closespacing of the insulating washers since the method by which the cablehas thus lbeen built or straightened in situ has eliminated 4 thenecessity for such clfse spacing. As above explained, this 'results ii.furtherreduction of attenuation due to the lower capacity and leakageconductance of the washers.

It will further be noted that the outer conductor consists of a seriesof completely closed loops uniformly low in resistance whose axescoincide throughout with the axis of the cable. Therefore the cable isbetter shielded in that it provides a uniformly low resistance returnconductor. These improvements not only provide a cable with lower andnon-variable attenuation characteristics, but also result in a cablewith a very low noise level. Because of the more eiiicient shielding italso results in a complete absence of coupling of external fields. Thecable may therefore be attenuated to-a lower level, thereby permitting awider. spacing of repeaters. The construction of the cable by the methodherein disclosed, made as it is largely from commercially availablematerials, has the added advantage that the resulting cable costs only afraction approximately 20%) of the coaxial cables heretofore proposed orinstalled.

Figs. 9 to 11 illustrate the invention as applied to an aerial coaxialcable. Referring to Fig. 9, there is shown a pole line p carrying acable supporting strand or messenger 90, to which are secured cablehangers 9| through which the outer tubular conductor 50 is pulled. Thetube 50 may conveniently be pulled from a special lightweight reel 92rotatably supported by members 93 and 94 on each side of the reel, thelatter members being readily secured by means of clamps 95 t0 thelsupporting messenger 90. Since the cable reel 92 and copper tubingthereon are relatively light in weight, the reel and cable thereon mayeasily be hoisted by a line into position for mounting on the supportingmessenger 90.

When a desired length of the tube, such as a length sufficient to reachseveral spans of pole line, has been pulled in through the cablehangers, the tube is straightened to remove bends which may have beenintroduced therein, generallyin the manner in' which the undergroundtube 50 of Fig. 6 was straightened. As shown in Fig. 10, the tubegripping device 66 is secured by means of thelink 68 to one end of adynamometer 98 which is employed to determine the amount of pullingtension applied to the tube 50. In this connection it is also desirableto use such a dynamometer when stretching the underground conductor 50of Fig. 6, and also the inner conductor 5|. The dynamometer is securedat its other end to the shackle 'I3 of the chain hoist or other pullingdevice employed. The pulling device 10 may be anchored or secured in anysuitable manner, as to an adjacent pole by means of a wire rope or chain99, as shown in Fig. 1l), or it may be secured by clamping the rope orchain to the messenger 80, or by a line passing through a pulleyattached to the pole or to the messenger, the line being secured at anadjacent pole or other suitable object. Preferably, the pulling force isexerted on the cable at both ends of the section, generally in themanner shown in Fig. 6. The straightening operation removes the variousbends in the l cable which are indicated in Fig. 9, and after theoperation the'cable is straight and true except for the' slight sa whichis occasioned by reason of the sag of t e supporting messengerintermediate the points of its support on the pole line.

After the tube 50 has been pulled in place and straightened asaforesaid, any remaining kinks,

l bends or irregularities in the inner surface of the tube may beremoved by pulling a three-ball hammer through the tube in the mannerhereinbefore described with reference to the underground cable.

Fig. 11 illustrates the step ofpulling in the inner conductor and washerassembly. The inner conductor and washers 52 may be carried on a'speciallightweight reel IM supported from the cable messenger 90 in the mannershown. After the inner conductor and insulating washers are pulled intothe tube 50, the end of which is protected bythe bell-mouth guide l0,the conductor 5| is stretched in the manner hereinbefore described inconnection with the underground cable, to remove any kinks, bends orirregularities in the conductor. The result is that the aerial coaxialcable is truly symmetrical and exhibits the same desirablecharacteristics as the underground cable referred to above. By reason ofthe fact that bends and curves in the outer tube 5|) may be removed inthe manner disclosed hereinbefore, such tubing may be coiled on smallerreels than was heretofore possible, and even if the tubing has becomebent in certain places to an extent tending to produce an ellipticalconfiguration, the straightening and hammering operations restore thetubing at such places to a circula; configuration. Also, for certainpurposes and in certain types of circuits in which some departure fromsymmetry of the conductors of the cable may be permissible, the cablemay be assembled in a cable manufacturing plant, and then reeled andtransported to the place where it is to be installed. After a length ofthe cable has been pulled in place and cut oil, the ends of the innerconductor, if desired, may be removed a slight distance sufficient topermit the insertion of the stretching tool 55 into the ends of thetubing, and with a suitable amount of stretching force exerted on theouter tubular conductor, the cable may be caused to have suitablecharacteristics for the purpose intended.

This procedure may also be followed with the inner conductor in themanner hereinbefore referred to, and in such case a short piece of theouter tube may be cut away to enable wire gripping devices to engage theinner conductor for the purpose of straightening the same.

Figs. 12 to 14 illustrate a preferred form of a tube engaging andstraightening tool which may be employed for straightening the outerconductor. The tool comprises a hollow circular steel body portion |||Iwhich has an expanding casing formed by splitting the body portion toprovide expanding leaves Ill, this expanding casing being inserted ashort distance into the end of the tubing as shown in Figs. 12 and 13.On the expanding casing are formed a series of circular tapered teeth orgripping portions ||2. Within the expanding casing is a taperedexpanding nut or plug Ill, the nut having in therewith a threaded rodportion ||5 which is received within the threaded portion of a rotatableshank ||5, the shank having a pulling eye Il formed therein as shown inFig. 13. When the shank III is rotated in the proper direction, thiscauses the threaded rod ||5 to be advancedw through the threadedVportion of the shank III. from right to left as viewed in the figure,thereby pulling the expanding nut III into the expanding casing l I Iand forcing the teeth I I2 outwardly to engage and grip the innersurface of the tube 50, as shown in Fig. 12. While the teeth ||2 biteinto the inner surface of the tube 5l to a slight extent, thisordinarily will not aifect the characteristics of the cable, although ifdesired the end portion of the tube Il into which the expanding tube isinserted may be cut of! after the straightening operation is completed.It is to be understood that other suitable gripping devices may beemployed in lieu of the device shown in Figs. 12 to 14, although thisdevice is preferred since it does not deform the structure of the tubewhen a straightening stress is applied thereto.

The various sections of the coaxial cable, whether aerial orunderground, may be spliced together in any suitable manner. A preferredmethod of splicing the ends of the conductors 5| of the adjacentsections together is shown in Fig. 15, these ends being bothmechanically and electrically joined by means of a metal connector pin|20 which is inserted with a tight fit in holes drilled in the ends ofthe abutting sections of the conductors 5|. A small amount of solder isthen applied to the splice where the adjoining conductors abut, and thesplice further heated, and by capillary attraction the solder is causedto flow between the abutting ends and also between the adjoiningsurfaces of the pin |20 and the holes in the conductors 5l which receivethe pin. This forms a sweated joint which is both electrically andmechanically strong, and the joint causes no increase or change in thediameter or configuration of the inner conductor 5| at the point wherethe adjacent sections of such conductor are spliced together. fter theinner conductors 5| have thus been joined together, a co1 per sleeve|25, which is longitudinally split at |25', is opened up suillciently toslip the sleeve in place so that it abuts the two ends of the outertubes 50, after which a connector sleeve |20 is soldered at |21 to thetubes 50, and thus the inner surface of the outer tube is smooth. Asshown by dotted lines in the ilgure, the ends of the sleeve |25 are cutat an angle to cause the solder to flow in between the sleeve andadjoining tubes 50, generally by capillary attraction,` thereby toprovide a sweated moisture-prooi' and gas-proof joint.

The sweated joint disclosed in Fig. 15 provides a connection between theadjacent ends of the inner conductor 5| which is ideal from anelectrical standpoint, but if greater mechanical strength is requiredas, for instance, to enable spliced sections of the conductor to bepulled in.

' the abutting ends of the conductors may be 13 spliced portions are asstrong as the unspliced portions of the conductor. Moreover, the brazedjoint is ideal from an electrical standpoint, since it introduces noappreciable increase in resistance by reason of the spliced portions.The method of thus brazing the abutting ends of the conductors may bethe same as that employed in brazing the abutting endsof other metallicbodies, and suitable brazing methods are welllknown in the art. Whenmaking a brazed butt joint, the pin `i211 may either be used or omitted,as desired; when used, its principal advantage is that ,it enables theabutting ends of the conductor to be held in proper alignment during thebrazing operation, although other devices may be employed for thispurpose. The diameter and configuration of the conductor 5l will remainunchanged at the brazed portions, and thus there is no change either inthe impedance of the conductor 5| or in the capaci-ty effect between theouter surface of the conductor 5| and the inner surface of the tube 50,and there is no extension or protuberance of the conductor 5I whichwould tend to cause echo effects or other disturbances in the coaxialcircuit.

An importan-t advantage in the foregoing construction is that the outer,preformed copper tubing 50 is moisture-proof and gas-proof and thereforeit is not necessary to provide an outer sheath of lead or the like toprevent the entrance of moisture or loss of gas, as is required whenstrip, fllamentary or Woven conductors are employed for a concentriccable. The copper tubing, with the inner conductor and insulatingWashers therein, may be carried in underground cable ducts or in aerialcable hangers of standard construction, generally in the manner of thecnventional lead cables employed for telegraph and telephone work. Twoor more coaxial cables in accordance with the present invention may belaid side by side in ducts or in cable hangers or other supports withoutsubstantial interference between their respective circuits. Also, ifydesired, where signal or repeater power feed circuitsare employed, anouter sheath of lead or other suitable material may be used forenclosing one or more coaxial cables and also communication or powerfeed conductors, the outer sheath affording the necessary protection forthe additional conductors.

The present invention enables the use of an conductor and washers arepulled in place, and y appreciably decreases the undesirable capacity,leakage and attenuation characteristics of the cable, so that theperformance of the cable very closely approximates that of a theoreticalcoaxial system, and makes the cable adaptable for very high frequenciessuch as of the order of 4 or 5 megaoycles and higher. While the washerspacing may be of the order of from 4 to 8 inches, it will be understoodthat if a smaller Washer spacing is desirable, for example, a 2 or 3inch spacing, this can readily be attained, and conversely the washerspacing may be greater than 8 inches, depending upon the size of thecable, the nature ofthe inner conductor, and other factors. The outertube preferably is composed of soft drawn copper and the inner conductorypreferably is composed of soft drawn or lmedium hard drawn copper wire,the resistance of which is less than that of hard drawn copper, althoughit will be appreciated that hard drawn copper wire may be employed forthe inner conductor and that materials other than soft drawn copper maybe employed for the iiexible outer tubing.

There are other important advantages incident to stretching the outerconducting tubing. One is that the tubing may originally comprise softdrawn copper which is easy to handle due to its flexibility and is easyto stretch and straighten due to its ductility, and after the tubing isinstalled in its permanent location the stretching operationhereinbefore referred to Adrawn copper so that, after the stretching andstraightening operation, the tubing has a further desirablecharacteristic in that it has now been changed to a medium hard-drawncopper tube and this gives greater strength and rigidity to the tube andmaterially prevents collapse of the tube at any portion thereof due tomechanical forces which the cable may encounter in service.

Moreover, the step of stretching the outer tubing has an importantadvantage in that it enables the inner conductor and Washer assembly tobe pulled into the tubing in longer sections. If bends were left in .thetubing, the frictional resistance of the inner conductor and washerassembly as it is pulled in increases enormously with the length of theconductor because of the excessive friction between the peripheral edgesof the insulator pieces and the bent portions of the outertube, plus theunit linear frictional resistance. The pulling force required in certaininstances wouldexceed the ultimate tensile strength of the innerconductor or at least stretch the inner conductor unduly and thusundesirably reduce its diameter. Reduction of the diameter of the innerconductor in this manner also tends to cause stripping of the washers asthe conductor was pulled in. However, when the outer tubular conductoris stretched and straightened in the manner disclosed, the pulling-inforce required is reduced to an amount only necessary to overcome theunit linear frictional resistance. Thus, the pulling force required isapproximately proportional to the length of the conductor. The forcerequired for pulling in long lengths of the inner conductor neverreaches the point where it is excessive or where it tends to cause thedisadvantages mentioned above which would result from the necessity ofemploying a much greater pulling force.

While, as hereinbefore, set forth, the bends, kinks and irregularitiespreferably are removed from the outer and inner conductors of thecoaxial cable when it is in its permanent location, or in some otherlocation such that it may be pulled into place without introducingsubstantial bends and irregularities therein, it is possible that forcertain classes of service the cable may be manufactured at a plant andlengths of the cable reeled, transported and installed with all of itscomponent parts intact. son of the fact that, when the outer tubing wasstraightened and stretched at the manufacturing plant, it thus becamemedium hard drawn, and

By rea- 15 if relatively large reels are employed for transporting thelengths of cable, the likelihood or danger of collapse of the cable isgreatly lessened, since the medium hard drawn copper does not tend tocollapse, as is the case with soit drawn copper tubing.

`It will be understood that, even some slight bends or other slightirregularities are introduced into the cable structure, they may be of acharacter which would not introduce distortion in excess of apredetermined permissible amount in the coaxial circuit, the permissibleamount depend-- ing upon the nature ofthe service, the signalingfrequencies employed, the proximity of disturbing circuits, and otherfactors. For example, if it is desired to use a cable with frequenciesof less than approximately one million cycles, the echo effects andother disturbances due to irregularities in the cable structure are notso pronounced as they are in the higher frequencies of the order ofseveral megacycles and above. In this connection, the fact that both theinner and` outer conductors are spliced in such a manner as to maintainthe diameters of the spliced portions the same as the remainingunspliced portions of the conductors means that there are no echoeffects or disturbances introduced by reason of changes in diameter dueto splices throughout the entire range of frequencies for which thecable is adapted, including the lower frequencies as well as the veryhigh frequencies, so that any disturbances due to splicing is eliminatedas a factor, and this enables a certain amount of other irregularitiesto be tolerated in the lower frequency range.

While the tool illustrated in Figs. 12 to 14 is there shown as employedfor stretching and straightening the conductor, the character of thistool is such that it is also well adapted for pulling in the outertubing either through bridle rings in the case of aerial cables orthrough ducts in the case of underground cables, and this ,pulling-inoperation is, therefore, effected without distorting the copper tubing.The tool also has the advantage in that the extent to which theexpansible portions lli and H2 grip the tubing is proportional to theamount of pulling force required; in other words, in pulling in the tubeit is only necessary that the shank H6 be rotated so as to cause theportions H2 to firmly grip the inner surface of the copper tubing, andany additional force required is only that due to the force imposed onthe tool by the pulling-in operation, so that the force With which thetool engages the tubing at no time is greater than that necessary forpulling it in, or for stretching, and never reaches an excessive amount.The gripping action of the'tool is, therefore, always automaticallyadjusted to the pulling force required.

The cable may be filled with a suitable inert gas, such as nitrogen,which is maintained under pressure higher than atmospheric pressure inorder to prevent the entrance of moisture because of an injury to orfault in the outer tube 50. 'I'he gas also forms an ideal dielectricbetween the conductors of the cable and further provides means formaking tests in the outer conductor to indicate the presence of aninjury or fault. The gas may be introduced into the cable by means ofvalve connections generally in the manner disclosed in Patent No.1,998,766, issued April 23, 1935.

In order to sectionalize or terminate the cables, cable dams may beemployed such, for example, as disclosed in Patents Nos. 1,769,524 and2,293,-

16 155. The provision of cable dams enables the cable readily to betested for defects in the sheath and splices. Various arrangements andsignal systems may be employed for locating defects in the gas-filledcable, such systems being disclosed in Patents Nos. 2,071,698 and2,219,262.

While, as above set forth, the bends, kinks and irregularitiespreferably are removed from the outer and inner conductors of thecoaxial cable when it is in its permanent location, or in some otherlocation such that it may be pulled into placewithout the danger ofintroducing bends and irregularities therein, it is possible that forcertain classes of service the cable may be manufactured at a plant andlengths of the cable reeled, transported and installed with all of itscomponent parts intact, particularly if sum.'- ciently large reels areemployed for storing and transporting the lengths of cable. In thisconnection, it is possible that if some slight bends or other slightirregularities are introduced into the cable structure, they may be of acharacter which would not introduce distortion in excess of apredetermined permissible amount in the coaxial circuit, the permissibleamount depending upon the nature of the service, the signalingfrequencies employed, the proximity of disturbing circuits, and otherfactors.

For brevity in the claims the expression installation is employed in ageneric sense to define the cable assembly in its permanent location orin some other location such that it may be transported and installed inits permanent location without substantially affecting deleteriously thedesired condition or characteristics thereof recited in the claims;similarly, the expression in situ" is employed in a generic sense todefine the permanent location of the cable or some other location suchthat the cable may be transported and installed in its permanentlocation without substantially afl'ecting deleteriously. the desiredcondition or characteristics thereof recited in the claims; theexpression flexible applied to the inner and outer conduc- .tor elementsof the cable means that these elements, as opposed to rigid, aresufficiently nonrigid as to be capable of being coiled or placed onreels for storage, transportation or other purposes; and the expressionlong lengths" employed means lengths sufficiently long for thepracticable construction of the cable, for example, lengths adapted toreach between manholes in the case of underground cable construction orbetween poles in the case of aerial pole line construction.

It will be evident to those skilled in the art to which the presentinvention pertains, that the invention is not limited to the specincmethods and structures shown and described, and that various changes andmodifications therein may be made, and I therefore do not desire to belimited to the precise method and structure shown and described exceptin accordance with the appended claims.

What is claimed is:

1. In the method of making a high frequency coaxial cable having aflexible outer conducting tube that encloses a exible inner centralconductor spaced from the outer tube by insulating pieces through whichthe central conductor passes, the step which comprises applying to saidouter tubular conductor a tensional force sumcient to uniformhr stretchthe tubular conductor to remove therefrom bends of a character whichwould introduce distortion in excess of a predetermined permissibleamount in the coaxial circuit.

2. In the method of making a high frequency coaxial cable having aflexible outer conducting tube that encloses a `flexible inner centralconductor spaced from the outer tube by insulating pieces through whichthe central conductor passes, the step which comprises applying to anend of the outer. tubular conductor a tensional force sufficient touniformly stretch the tubular conductor to remove therefrom bends of acharacter which would introduce distortion in excess of a predeterminedpermissible amount in the coaxial circuit.

3. In the method of making a high frequency coaxial cable having aflexible outer conducting f.

tube that encloses a flexible inner central conductor spaced from theouter tube by insulating pieces through which the central conductorpasses, the steps which comprise applying to the outer conductor atensional force sufcient to unformly stretch said outer'conductor so asto remove therefrom bends of a character which would introducedistortion in excess of a predetermined permissible amount in thecoaxial circuit. and applying a tensional force to said inner conductorto uniformly stretch said inner conductor to remove bends and kinkstherefrom.

4. The method of making a high frequency coaxial cable having a flexibleouter conducting tube that encloses a flexible inner central conductorspaced from the outer tube by insulating pieces through which thecentral conductor passes, which comprises securing to 'the centralconductor in predetermined spaced relation longitudinally of theconductor insulator pieces of slightly less diameter than the innerdiameter of the outer tube, applying to the outer tube a' ,tensionalforce suflicient to uniformly stretch the outer tube to remove therefrombends of a character which would introduce a distortion in excess of apredetermined permissible amount in the coaxial circuit, drawing thecentral conductor and insulator pieces secured thereto through saidouter tube until the inner conductor and spacing pieces are properlypositioned within the outer tube, and then applying a tensional force tosaid inner conductor to uniformly stretch the inner conductor untilbends or irregularities therein are smoothed out.

5. The method of making a high frequency coaxial cable having a flexibleouter conducting tube that encloses a flexible inner central'conductorspaced from the outer tube by insulating pieces through which thecentral conductor passes, which comprises securing to the centralconductor in predetermined spaced relation longitudinally of theconductor insulator pieces of slightly less diameter than the innerdiameter of the outer tube, applying to the outer tube a tensional forcesucient to uniformly stretch the tube to remove therefrom bends of acharacter which would introduce distortion in excess of a predeterminedpermissible amount in the coaxial circuit, passing a shaping devicethrough said tubing to remove kinks and irregularities from the innersurface of said tubing, subsequently drawing the central Iconductor andinsulator pieces secured thereto through said outer tube until the innerconductor and spacing pieces are properly positioned Within the outertube, and then applying a tensional force to .said inner conductor touniformly stretch the inner conductor until bends or irregularitiestherein are smOQihed out.

6. The method of making a high frequency coaxial cable having a,flexible outer conducting tube that encloses a flexible inner centralconductor spaced from the outer tube by insulating pieces through whichthe central conductor passes, which comprises securing to the centralconductor in predetermined spaced relation longitudinally of theconductor insulator pieces of slightly less diameter than the innerdiameter of the outer tube, installing in situ a section of the outertube, applying to the section thus installed a tensional forcesufficient to uniformly stretch the section to remove therefrom bends ofa character'which would introduce distortion in excess of apredetermined permissible amount in the coaxial circuit, passing ashaping device through l said section of the tubing to remove kinks orirregularities from the inner surface of the tubing, drawing the centralconductor and insulator pieces secured thereto through said installedsection of the tubing until the inner conductor and spacing pieces areproperly positioned within the outer tube, and applying a tensionalforce to the inner conductor to uniformly stretch the conductor thusinstalled to remove kinks and irregularities in the inner conductor.

'1. A high frequency coaxial cable comprising a preformed flexible outerconducting tubing that forms one of the conductors of the coaxialcircuit, the tubing substantially comprising a material that is capableof being uniformly stretched, anA

inner conductor centrally positioned within said tubing, said tubingbeing substantially free of bends of a character which would causesignal distortion in excess of a predetermined permissible amount in thecoaxial circuit, said condition hav' ing been obtained by theapplication to the outer tubular conductor of a tensional forcesufficient to stretch the same uniformly and remove therefrom bends ofthe aforesaid character which had been introduced into said conductor.

8. A high frequency coaxial cable comprising a` flexible, seamlesscopper tubing that forms one of the conductors of the coaxial circuit,an inner conductor centrally positioned within said tubing, said tubingbeing substantially free of bends of a character which would causesignal distortion in excess of a predetermined permissible amount in thecoaxial circuit, said condition having been obtained by the applicationof a tensional force to an end of the tubing sufficient to stretch thesame uniformly and remove therefrom bends of the aforesaid characterwhich had been introduced into said tubing.

9. A high frequency coaxial cable comprising a preformed flexible outerconducting tubing that forms one of the conductors of the coaxialcircuit, the tubing substantially comprising a mate- `rial that iscapable of being uniformly stretched,

saidV tubing being substantially free of bends, kinks and irregularitiesof a character which would cause signal distortion in excess of apredetermined permissible amount in the coaxial circuit and having asmooth inner circular configuration of uniform diameter substantiallythroughout the length thereof, said condition having been obtained bythe application of a tensional force to the outer conductor sumcient tostretch the same uniformly and remove bends therefrom and by passage ofa shaping device through said tubing to remove kinks and irregularitiesfrom the inner surface thereof, an inner conductor installed within saidtubing, insulator pieces centrally supporting said inner conductorwithin the tubing, the inner conductor being subsignal distortion inexcess of a predetermined permissible amount in the coaxial circuit andhaving a smooth inner circular configuration of uniform diametersubstantially throughout the length thereof, said condition beingobtained by the application of a tensional force to the outer conductorsufilcient to stretch the same uniformly and remove bends therefrom andby passage oi a shaping device through said tubing to remove kinks andirregularities from the inner surface thereof, an inner flexible, copperconductor installed within said tubing, insulator pieces centrallysupporting said inner conductor within the tubing, the inner conductorbeing substantially free of bends, kinks and irregularities of acharacter which would cause signal distortion in excess of apredetermined permissibleamount in 20 the coaxial circuit. saidcondition having been obtained by the application of a tensional forceto the inner conductor suiiicient to stretch the same uniformly afterits installation in said outer tubing.

, ADOLPH Z. MAMPLE.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,129,712 Southworth Sept. 13,1938 2,199,045 Dallenbach Apr. 30, 1940 2,262,020 Llewellyn Nov. 11,1941 2,156,772 Seeley May 2, 1939 1,327,491 Pierce Jan. 6, 1920 622,792Bates Apr. 11, 1899 175,807 Wistar Apr. 4, 1876 2,057,242 Mautsch Oct.13, 1936 2,293,155 Mample Aug, 18, 1942 2,336,524 Bannister Dec. 14,1943 1,367,099 Schmidt Feb. 1, 1921 1,885,195 Green Nov. 1, 1932 FOREIGNPATENTS Number Country Date Germany Aug. 1928

